Steam announces the return of X-COM: Terror from the Deep to the online
delivery service, as well as the addition of four other titles from the highly
regarded UFO strategy series, X-COM: Apocalypse, X-COM: Enforcer,
X-COM: Interceptor, and X-COM: UFO Defense. The games are
available separately for $4.99 USD each, or as part of their
X-COM collection for $14.99 USD. During this first week of availability,
they are offering an additional 10% off those prices.

Which begs the question...can anyone answer if mods do indeed work on the Steam versions?

As far as I know, they will. Even mods that alter the exe should work, as I don't think 2K altered it (though they may have renamed it). Not 100% sure though, but guys like Zombie and Bomb Bloke at our forum will know (they're X-COM experts/maniacs ;)).